To produce a silver halide photographic material, in general, an aqueous solution of a silver salt, for example an aqueous solution of silver nitrate, and a solution of water-soluble halide which are a reacting agent (substance) are added to an aqueous solution of colloid, for example an aqueous solution containing gelatin to make silver halide grains precipitated therein. As means of adding these reacting agents to the aqueous solution of colloid, for example, a single jet method and a double jet method are widely known. Such reaction means are disclosed, for example, by Pierre Glafkides in Photographic Chemistry (published by Fountain Press, London, 1958), pp. 327 to 330, and by T. H. James in The Theory of the Photographic Process, Vol. 4, pp. 88 to 104.
The photographic properties of the silver halides to be produced according to the above-mentioned methods for producing photographic emulsions are determined essentially depending on the stirring condition for precipitating or physically ripening the grains. The performance of the stirrer to be used for such purposes, including, for example, the bulk mixing time (this is defined as the time necessary for mixing the aqueous solution of silver nitrate that has been pulse-wise added to a reactor with the aqueous solution of colloid until the plural silver electrodes provided in the reactor become uniform), the critical growth speed (this is defined as the maximum addition speed at which silver halide grains are grown in the presence of seed crystals without forming any new nucleus), etc. is determined substantially by the flow rate to be jetted out from the stirrer. The flow rate is defined as the amount of the flow to be discharged from the stirrer within one minute by the action of the stirring blades in the stirrer. The flow rate (Q) to be jetted out from a stirrer (i.e., the delivery rate) is generally represented by: EQU Q.about.Nd.sup.3
(i.e., Q=N.sub.Q Nd.sup.3 wherein N.sub.Q means a delivery coefficient) wherein N means the stirring rotation number of the blades in the stirrer, and d means the diameter of each blade therein. Therefore, the flow rate is proportional to the stirring rotation number of the blades in a stirrer. This is described in, for example, Practice in Stirring Technology (edited by the Association for Technology Information of Japan, 1989), page 42.
Further, the above relation is in detail described in N. Harnby, M. F. Edwards, A. W. Nienon, Mixing in the Process Industries.
Therefore, in order to improve the general performance of a stirrer, it is necessary to rotate the stirring blades therein at high speed when the diameter of each stirring blade therein is defined to a constant value. Being different from a side-arm-type closed stirrer such as that described in JP-B-48-21045 (the term "JP-B" as referred to herein means an "examined Japanese patent publication"), a submerging type stirrer having a large opening area naturally has a small stirring rotation number (about several hundreds to 3000 rpm or so) because of the problem of foaming and, therefore, even the slight increase in the stirring rotation number results in a noticeable increase in the performance of the stirrer itself. For these reasons, it is desired that such a submerging type stirrer is stirred at the greatest possible speed.
If the stirring rotation number of an axial-flow-type stirrer that generates a flow in the direction parallel to the stirring axis, such as that described in JP-B-35-10545, is increased so as to enhance its mixing performance, the stirrer thereby generates a V-cut flow due to the enlarged horizontal rotation flow. The V-cut flow is caused by the centrifugal force of the horizontal rotation flow that makes the liquid in the stirrer pushed toward the side wall of the stirrer, on the contrary, making the level of the liquid near the stirring axis depressed to have a V-cut shape. The V-cut flow causes foaming of the liquid being stirred, since it takes air in itself while it runs from the side wall of the container to the stirring axis.
If the stirring rotation number of a radial-flow-type stirrer that generates a flow in the radial direction perpendicular to the stirring axis, such as that described in JP-B-49-48964, is increased so as to enhance its mixing performance, the stirrer thereby generates a mountain-like flow that expands near the side wall of the stirrer. Such a mountain-like flow also causes foaming of the liquid being stirred, since it takes air in itself while it runs from the side wall of the stirrer to the center thereof.
The foaming of the liquid being stirred, that is caused by the increase in the stirring rotation of such a stirrer, often involves various problems in that the liquid gives coarse grains thereby broadening the grain size distribution of the silver halide grains formed, that the critical growth speed of the silver halide grains being formed is lowered, that the photographic properties of the grains formed are worsened (for example, the emulsion comprising the grains is easily fogged or the contrast of the emulsion is lowered), and that the stability and the efficiency in producing the grains are lowered. In particular, the foaming in question is especially problematic in a so-called controlled double jet method for producing a silver halide emulsion while controlling the silver potential of the emulsion, which is most popularly employed in this technical field in these days, in that the foams generated in the emulsion adhere to the silver electrode thereby making it impossible to accurately measure the silver potential of the emulsion being formed and making it difficult or impossible to control the silver potential. The pollution of various sensors such as the silver electrode, etc. by the foams generated is noticeable also during the step of desalting the emulsion formed. To remove the problems to be caused by the foaming, generally employed are a means of adding a coagulant-to the liquid being stirred in a stirrer by which a part of the coagulated solids (grains having a small specific gravity or small grains) is made floated on the surface of the liquid without being precipitated, and a means of submerging a drain duct having a strainer at its tip in the supernatant formed over the liquid being stirred, through which the supernatant is removed by means of a pump. According to these means, however, the strainer used is clogged by the coagulated solids floating over the liquid along with the foams.
To overcome the above-mentioned problems, there is known defoaming technology such as physical and mechanical defoaming and chemical defoaming in terms of the practical performance. If the former physical and mechanical defoaming means is applied to the axial-flow-type stirrer that is often troubled by the foaming mechanism mentioned above, the stirrer must be so reconstructed that it may inhibit the horizontal rotation flow. If, on the other hand, it is applied to the radial-flow-type stirrer, the stirrer must be so reconstructed that it may inhibit the mountain-like flow near the side wall of the stirrer. Thus, the physical and mechanical defoaming means requires the reconstruction of the stirrer, depending on the type of the stirrer to which it is applied. For instance, a conventional physical and mechanical defoaming means applied to an axial-flow-type stirrer has been proposed in JP-B-57-92524 and Japanese Utility Model Publication No. 62-16183, in which a flow-controlling plate is fixed horizontally at the top of the stirrer by which the horizontal rotation flow generated is converted into a perpendicular flow thereby inhibiting the generation of the V-cut flow. On the other hand, a conventional physical and mechanical defoaming means applied to a radial-flow-type stirrer has been proposed in Japanese Patent Application No. 6-11675, in which a dimple structure or a projection structure having many dimples or projections is disposed on at least a part of the inside wall of the stirrer or in at least a part of the internal space of the stirrer. Both of these conventional physical and mechanical defoaming means are effective in terms of only the foam-inhibiting object but are still defective in terms of the object of rapidly extinguishing the foams (that is, the object of shortening the time necessary for extinguishing the foams) when the stirring of the liquid in the stirrer is stopped. The drawback of the means having such a poor ability to rapidly extinguish the foams is still problematic, as worsening the production stability and efficiency.
Various chemical defoaming means of using defoaming agents have heretofore been known in various industrial fields. As such defoaming agents, in general, used are silicone emulsions, sorbitan fatty acid esters, higher alcohols, animal and vegetable oils, mineral oils (e.g., paraffin, etc.), polyoxyalkylene glycol derivatives, etc. Examples of these are described in Practice in Stirring Technology (edited by the Association for Technology Information of Japan, 1989), page 121.
Various defoaming agents such as those mentioned below are used in the production of emulsions of silver halide grains.
In U.S. Pat. No. 5,147,771, European Patent 513,723A, U.S. Pat. Nos. 5,147,772 and 5,147,773, there is disclosed the use of polyethylene oxide block copolymers to obtain good monodispersed emulsions. Of the disclosed copolymers, those having a propylene oxide content (PO) of 80% or more are effective as the intended defoaming agent. As commercial products, effective are Pururonic 31R1, 25R2, 17R2, L101 and L61 (all products of BASF), and Tetronic 1301, 901, 130R1 and 110R1 (all products of BASF).
In JP-B-44-9497, JP-B-44-26580, JP-A-59-188640, JP-A-59-189339, JP-A-62-231246 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), described are various defoaming agents such as copolymers of polypropylene oxide (PPO) and polyethylene oxide (PEO) having a PPO content falling within a defined range, water-soluble alkaline earth metals, lower alcohols, etc.
Copolymers of PPO and acrylamide having a PPO content of 70% or more and mixtures of side-chained PPO homopolymer(s) and side chained PEO homopolymer(s) having a PPO content of 70% or more, such as those described in JP-A-7-98482 and EP063349A1 (corresponding to JP-A-7-28183) are effective as the defoaming agent intended herein.
When the above-mentioned defoaming agents are used, in general, attentions shall be paid to the following matters. Alcohols such as 2-ethylhexyl alcohol, octyl alcohol, etc. exhibit the defoaming effect only when they have just added to a liquid to be defoamed. However, their defoaming effect is insufficient if a relatively large amount of them is not added. In the factory where photographic materials are produced generally in a closed system, it is difficult to treat vaporized alcohols. In addition, the foam-inhibiting effect of the alcohols is not good. Silicone oils exhibit an excellent defoaming effect. However, when emulsions containing such a silicone oil are coated on supports, the supports often repel the emulsions due to the silicone oil contained therein whereby the quality of the coated supports (photographic materials) is often lowered. Therefore, special attention shall be paid to the handling of silicone oils. There is a probability that alkylene oxide copolymers are often deteriorated in the photographic properties of silver halide photographic emulsions. Therefore, special attention shall be paid to the amount of the copolymers, if used. In particular, the amount of the copolymer to be used for defoaming surfactants having a large HLB value (hydrophobic/hydrophilic balance) is about 10 times the amount thereof to be used for defoaming proteins such as gelatin, etc. If such a large amount of the copolymer is added to a photographic emulsion, it causes competing adsorption against the sensitizers to be in the emulsion while having in no small way some negative influences on the photographic properties of the emulsion to lower the sensitivity of the emulsion or to increase its fog. The above-mentioned commercial products, Pururonic have some negative influences on the formation of silver halide grains, depending on the condition for the formation. For example, if Pururonic TM31R1 is used for the formation of tabular silver iodobromide grains, it makes the grains thick thereby lowering the aspect ratio of the grains. In addition, it is adsorbed to the (100) face of the grains to retard the growth of the grains. Moreover, it depends on pH values. As mentioned hereinabove, there are various problems when the conventional defoaming agents are applied to the formation of silver halide grains to be in photographic emulsions.